Cellular terminal location using GPS signals in the cellular band

ABSTRACT

Aspects of global positioning system (GPS) technology and cellular technology are combined in order to provide an effective and efficient position location system. In a first aspect of the invention, a cellular network is utilized to collect differential GPS error correction data, which is forwarded to a mobile terminal over the cellular network. The mobile terminal receives this data, along with GPS pseudoranges using a GPS receiver, and calculates its position using this information. According to a second aspect, when the requisite number of GPS satellites are not in view of the mobile terminal, then a GPS pseudosatellite signal, broadcast from a base station of the cellular network, is received by the mobile terminal and processed as a substitute for the missing GPS satellite signal. A third aspect involves calculating position using GPS when the requisite number of GPS satellites are in view of a GPS receiver, but when the requisite number of GPS satellites are not in view of the GPS receiver, then position is calculated using the cellular network infrastructure. When the requisite number of GPS satellites come back into view of the GPS receiver, then position is again calculated using GPS. A fourth aspect involves using cellular signals already being transmitted from base stations to terminals in a cellular network to calculate a round trip delay, from which a distance calculation between the base station and the terminal can be made. This distance calculation substitutes for a missing GPS satellite signal.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to global positioning satellitesystems and cellular networks, and in particular, combining aspects ofeach technology in order to provide an efficient, reliable, and highlyaccurate position location system.

BACKGROUND OF THE INVENTION

[0002] The NAVSTAR (Navigation System with Time and Range) GlobalPositioning System (GPS) is a space-based radio-positioning andtime-transfer system. While the system was originally developedprimarily for military purposes, it now also contains a “coarseacquisition” (C/A) channel that is available for general civilian use.GPS provides accurate position, velocity, and time (PVT) information fora given object anywhere on the face of the earth, such as a movingmobile terminal in a vehicle. The NAVSTAR GPS includes three majorsystem segments: (i) a space segment, (ii) a control segment, and (iii)a user segment. Briefly, the space segment has twenty four NAVSTARsatellites, each of which broadcasts radio frequency (RF) ranging codesand navigation data messages. Each navigation data message includes suchdata as satellite clock-bias data, ephemeris data (precise orbital dataof the satellite), certain correction data, and satellite almanac data(coarse orbital data on the 24 satellites). The twenty four satellitesare arranged in six orbital planes with four satellites in each plane,and the orbital planes are inclined at an angle of 55 degrees relativeto the earth's equator. The control segment primarily consists of amaster control station currently at Falcon Air Force Base in Colorado,along with monitor stations and ground antennas at various locationsaround the world. The master control station monitors and managessatellite constellation. The monitor stations passively track GPSsatellites in view and collect ranging data for the satellites. Thisranging data is transmitted to the master control system where satelliteephemeris and clock parameters are estimated and predicted. Furthermore,the master control system uses the ground antennas to periodicallyupload the ephemeris and clock data to each satellite for retransmissionin the navigation data message. Finally, the user segment comprises GPSreceivers, specially designed to receive, decode, and process the GPSsatellite signals.

[0003] Generally, the satellites transmit ranging signals on two D-bandfrequencies: Link 1 (L1) at 1575.42 MHz and Link 2 (L2) at 1227.6 MHz.The satellite signals are transmitted using spread-spectrum techniques,employing ranging codes as spreading functions, a 1.023 MHz coarseacquisition code (C/A-code) on L1 and a 10.23 MHz precision code(P-code) on both L1 and L2. The C/A-code consists of a 1023 bitpseudorandom (PRN) code, and a different PRN code is assigned to eachGPS satellite, as selected from a set of codes called Gold codes. TheGold codes are designed to minimize the probability that a receiver willmistake one code for another (i.e., minimize cross-correlation). TheC/A-code is available for general civilian use, while the P-code is not.In addition, a 50 Hz navigation data message is superimposed on theC/A-code, and contains the data noted above.

[0004] In particular, the navigation message has 25 frames of data, eachframe having 1,500 bits. Each frame is divided into five subframes of300 bits each. At the 50 Hz transmission rate, it takes six seconds toreceive a subframe, thirty seconds to receive one data frame, and 12.5minutes to receive all twenty five frames. Subframes 1, 2, and 3 havethe same data format for all twenty five frames. This allows thereceiver to obtain critical satellite-specific data within thirtyseconds. Subframe 1 contains the clock correction for the transmittingsatellite, as well as parameters describing the accuracy and health ofthe broadcast signal. Subframes 2 and 3 contain ephemeris parameters.Finally, subframes 4 and 5 contain data common to all satellites andless critical for a receiver to acquire quickly, namely almanac data andlow-precision clock corrections, along with other data.

[0005] The ranging codes broadcast by the satellites enable the GPSreceiver to measure the transit time of the signals and therebydetermine the range between the satellite and the receiver. It should benoted, however, that range measurements inherently contain an errorcalled an offset bias common to all the measurements created by theunsynchronized operation of the satellite and the user's clocks. SeeU.S. Pat. No. 5,467,282 to Dennis. This user clock error will yield anerroneous range measurement, making it appear that the user is eithercloser to or farther from each of the satellites than is actually thecase. These measurements are therefore more accurately termedpseudoranges. The navigation data messages enable the receiver tocalculate the position of each satellite at the time the signals weretransmitted.

[0006] In general, four GPS satellites must be in clear view of the GPSreceiver in order for the receiver to accurately determine its location.The measurements from three GPS satellites allow the GPS receiver tocalculate the three unknown parameters representing itsthree-dimensional position, while the fourth GPS satellite allows theGPS receiver to calculate the user clock error, and therefore determinea more precise time measurement. The GPS receiver compiles thisinformation and determines its position using a series of simultaneousequations.

[0007] In addition, when the GPS receiver is first turned on, it mustcalculate its initial position. This initial determination is known as a“first fix” on location. Typically, the receiver must first determinewhich satellites are in clear view for tracking. If the receiver is ableto immediately determine satellite visibility, the receiver will targeta satellite and begin its acquisition process. If there is no almanac orposition information already stored in the receiver, then the GPSreceiver enters a “search the sky” operation that searches forsatellites. Once the satellites are tracked, the receiver beginsreceiving the necessary data, as described above.

[0008] The “time-to-first-fix” (TTFF) represents the time required for areceiver to acquire the satellite signals and navigation data, and tocalculate its initial position. If the receiver has no estimate ofcurrent time and position and a recent copy of almanac data, then thisprocess generally takes about 12.5 minutes, which is the time necessaryto receive a complete navigation data message assuming a 50 Hztransmission rate and receipt of twenty five frames of data, asdescribed above.

[0009] A common problem with the conventional GPS is not having four GPSsatellites in clear view of the GPS receiver. This commonly arises, forexample, in a city setting such as in an urban canyon—i.e., in theshadow of a group of tall buildings—which can block the GPS satellitesignals, or indoors in the buildings themselves. In such situations, theGPS receiver is unable to accurately determine its location using GPS.

[0010] Therefore, the need arises to find a replacement for the one ormore missing GPS satellite signals. One method for accommodating thisproblem is to provide pseudosatellite signals that are transmitted inthe GPS frequency band. They provide much the same information that thetypical GPS satellite does, and are utilized by the GPS receiver in muchthe same fashion as the typical GPS satellite signal. These signals mayoriginate from dedicated stations that are located on the ground atstrategic locations, such as at airports. However, pseudosatellitesignals are stronger than the GPS satellite signals and therefore, blockthe GPS signals. Thus, they generally transmit for only ten percent ofthe time. That is, they transmit periodically, known as burst mode, suchas on for ten percent of the time and off for ninety percent of thetime.

[0011] In addition to drowning out actual GPS satellite signals, theconventional pseudosatellite signal approach has other disadvantages.For one, there is the need to have specialized dedicated stations atstrategic locations to transmit this information. This increases thecost of the GPS, and requires the need for obtaining permission from thelandowner to set up and operate such dedicated stations. In addition,the user must be located within some specified distance of the stationin order to receive the pseudosatellite signal, which is not always thecase. Therefore, there is a need for a more efficient, less costly, andreliable alternative for addressing the situation of an inadequatenumber of GPS satellites being in clear view of the GPS receiver.

[0012] In addition, even when four satellites are in view, and the GPSreceiver is readily receiving all of the necessary pseudorange data forcalculating its position, there are further common errors present thatresult in erroneous position determinations. These errors includephysical errors such as signal path delays through the atmosphere, i.e.,propagation signal delay, and satellite clock and ephemeris errors. Inaddition, for civilian users, the Government introduces errors fornational security reasons, generally known as selective availabilityerrors (SA). SA primarily includes ephemeris data error and clock error,and results in an erroneous position determination of approximately 25to 100 meters.

[0013] In order to help reduce the effects of these errors, adifferential GPS (DGPS) may be employed. DGPS can achieve accuracies inthe order of ten meters. The typical DGPS architecture includes one ormore reference stations at precisely known, fixed reference sites, andDGPS receivers. The reference station includes a reference receiverantenna, a differential correction processing system, and data linkequipment. As an example, the United States Coast Guard has set upreference stations that broadcast the differential correction data,which is typically used by ships.

[0014] There are two primary variations of the differential measuringtechniques. One technique is based on ranging-code measurements and theother is based on carrier-phase measurements. In general, theranging-code differential technique uses the pseudorange measurements ofthe reference station to calculate pseudorange or position correctionsfor the user receivers. The reference station calculates the pseudorangecorrections for each visible satellite by subtracting the “true” range,determined by surveyed position and the known orbit parameters, from themeasured pseudorange. The reference station typically broadcasts thepseudorange corrections in real-time on a low frequency beacon channel,which is received in real-time by the DGPS receiver. Of course, both theDGPS receiver and the reference receivers could alternatively collectand store the necessary data for later processing. The DGPS receiverselects the appropriate correction for each satellite that it istracking, and subtracts the correction from the pseudorange that it hasmeasured. For example, with the reference station set up by the CoastGuard, the station will broadcast the pseudorange corrections as radiosignals. Ships having DGPS receivers receive this radio signal andprocess it to correct the pseudorange data obtained from the GPSsatellites.

[0015] The other differential technique is the carrier-phasedifferential technique, which is typically used in applicationsrequiring high accuracy such as in surveying or for an aircraft landingsystem. This method measures the difference in phase of the carrier atthe reference and mobile unit. The ambiguity in the integer number ofcycles is determined by either bringing the antennae of the referenceunit and mobile unit close together (less than one wavelength), or byredundant measurements and complex search algorithms to determine thecorrect solutions.

[0016] Furthermore, DGPS may be designed to serve a limited area from asingle reference station, which is generally called a local area DGPS(LADGPS). In the alternative, the system may use a network of referencestations and known algorithms to extend the validity of the DGPStechnique over a wide area—known as Wide Area GPS, or WADGPS.

[0017] The typical DGPS presents certain drawbacks. One drawback is thatthe DGPS must use its own frequency band, so as not to interfere withthat of the stand alone GPS. In addition, the DGPS receiver presents anadditional receiver that must operate independent of the GPS receiversin receiving the differential correction data. These problems work indirect tension with the desire to make such systems as small and compactas possible, with as little additional circuit structure as possible,and still be as efficient as possible in terms of utilizing limitedfrequency.

[0018] Another area of interest for the present invention is cellulartechnology. FIGS. 1 and 2 show a typical cellular network, and its maincomponents. See U.S. Pat. No. 5,546,445 to Dennison et al. The typicalcellular network 100 covers a contiguous area that is generally brokendown into a series of cells 110. Each cell has a base station 210 thatmaintains communication with the mobile terminal 220 (e.g., a cellularphone). The base station 210 includes a transmitter and receiver (ortransceiver), and an antenna that transmits a wireless signal over agiven area. The transmit power of the base station is directly relatedto the size of the cell, where the greater the transmit power of thebase station, the larger the size of the cell.

[0019] The overall management of the cellular system is handled by amobile telecommunications switching office (MTSO) 120. The MTSO providesnumerous functions for the cellular system, such as assigning calls to acell based on availability and signal strength, call statistics, andbilling for the cellular network. The MTSO also functions as theinterface between the cells and the Public Telephone Switching Network(PTSN) 140 for connection to the local telephone company 230 and longdistance toll centers.

[0020] In configuring the cellular network, the desired size of the celldepends on the geographic nature of the coverage area and the amount oftraffic expected in that area. Each cell uses a group of assignedfrequencies or channels. In addition, where traffic becomes too heavy ina given area, the cell may be split into smaller cells by a processknown in the art as “cell splitting.” This concept is generallyillustrated in FIG. 1.

[0021] In many instances, a cellular user also wishes to determine theirlocation. The cellular user may carry around a GPS receiver fordetermining location. An alternative is to have the GPS receiverincorporated into the cellular mobile terminal. See, for example, U.S.Pat. Nos. 5,043,736 to Darnell et al and 5,625,668 to Loomis et al.Methods also exist for determining location in a cellular systemindependent of GPS in order to determine location, such as using thecellular network infrastructure. Two examples for calculating position(though not the only methods) are (i) using Time Of Arrival (TOA)measurements when the time of transmission of the signal from the basestations is known, or (ii) using Time Difference of Arrival (TDOA)measurements when the actual time of transmission is not known, butperiodic signals are available, as explained below.

[0022] Referring generally to FIG. 3, a typical urban street pattern 300is shown to illustrate the first method of using TOA measurements. Whenthe time of transmission of the signal from a base station 310 is known,a mobile terminal 320 simply determines when that transmitted signal isreceived. The difference in time from transmission to receipt, alsoknown as the propagation delay, multiplied by the speed of light,provides a radial distance measurement R between that base station andthe mobile terminal. Calculating the distance between the mobileterminal and three different base stations provides an accurate locationfix for the mobile terminal, as the intersection of three spheres.

[0023] In the second method of utilizing TDOA measurements, while thisapproach can also be used when the actual time of transmission of thesignal from the base stations is available, it may also be used whensuch time of transmission is not available, but periodic signals are.This may occur with some cellular systems. Some CDMA (Code DivisionMultiple Access) systems, such as those conforming to the IS-95standard, do provide transmissions at well defined times.

[0024] The periodic signal entails each of the base stationstransmitting periodic signals that are synchronized with one another. Inthat regard, all of the base stations may transmit their periodicsignals at the same exact time, or with some specified timing offsetbetween base stations. In this method, the mobile terminal measures thedifference in time between the arrival of a signal from one base stationwith respect to another. This time difference of arrival (TDOA),together with the known locations of the two base stations and the speedof radio signal transmission, defines a hyperbolic surface with the basestations at the foci. The mobile terminal's location is somewhere onthis surface. Thus, a single TDOA measurement does not uniquely definethe location of the mobile terminal. However, a similar measurement forsignals from other pairs of base stations defines additional surfaces.By measuring the TDOA of the signals from three base stations, threesurfaces can be determined, the common intersection of which establishesthe location of the mobile terminal.

[0025] Further information and systems regarding conventional TDOAlocation systems and methods may be found in Krizman et al., “WirelessPosition Location Fundamentals, Implementation Strategies, and Sourcesof Error,” presented at the IEEE Conference on Vehicular Technology,Phoenix, Ariz., May 5-7, 1997 and in the issue of the IEEECommunications Magazine, Apr. 1998, Vol. 36, No. 4, pages 30-59. Theentirety of this reference is hereby incorporated into the presentdisclosure for its teachings regarding conventional TDOA locationmethods and systems.

[0026] However, problems exist with using these two methods fordetermining location. One significant problem results from multi-patherrors. Such errors result from changes in the transmission path of thesignal that the mobile terminal receives from the base station. Forexample, when the user of the mobile terminal goes around a corner, themobile terminal may receive a new signal from the base station that hasfollowed a completely different transmission path compared to the oldsignal that the mobile terminal was previously receiving before the userturned the corner. Therefore, the distance traveled by the signal willlikely differ. This causes a change in time measurement by the mobileterminal that does not accurately represent the actual distance changeof the mobile terminal from the base station, thereby rendering aninaccurate location determination by the mobile terminal.

[0027] Another problem encountered is that the typical clock in acellular mobile terminal does not measure time precisely, and may have atendency to drift, generally known as clock drift. Therefore, the timemeasurements made by the terminal are not extremely accurate, whichresults in an erroneous time—and therefore location—determination. Theerror due to the drift grows larger the longer the mobile terminal clockis used.

[0028] In sum, as shown above, a need exists for a more efficient andless costly structure compared to the conventional DGPS receiver. Inaddition, a need exists for more efficient, reliable, and effectivesolutions to address the problem of receiving an inadequate number ofsatellite signals from the GPS satellites.

SUMMARY OF THE PRESENT INVENTION

[0029] It is an object of the present invention to provide a system thatcombines GPS and cellular technology in order to overcome deficienciesassociated with the use of either technology alone, in order to providea more efficient, reliable, and effective position determination for agiven object such as a mobile terminal.

[0030] It is a further object of the present invention to provide aposition location system that utilizes the cellular network to forwardDGPS error correction information to a mobile terminal.

[0031] It is yet another object of the present invention to provide aposition location system that efficiently utilizes the cellularfrequency band available in a cellular network for forwarding DGPS errorcorrection information to a mobile terminal.

[0032] It is a further object of the present invention to provide aposition location system that provides a cellular network with thecapability of receiving and forwarding DGPS error correction informationthat is utilized by the mobile terminal for accurately determining itsposition.

[0033] It is another object of the present invention to provide aposition location system that provides a cellular network with thecapability of receiving and forwarding DGPS error correction informationto a DGPS processor, and also forwarding GPS pseudoranges from themobile terminal to the DGPS processor, wherein the GPS pseudoranges arecorrected.

[0034] It is yet another object of the present invention to provide aposition location system that compensates for the inability of a mobileterminal containing a GPS receiver to view the requisite number of GPSsatellites to obtain an accurate fix on its location.

[0035] It is yet a further object of the present invention to provide aposition location system that utilizes the base station of a cellularnetwork to transmit GPS pseudosatellite signals such that if therequisite number of GPS satellites are not in clear view of the mobileterminal containing the GPS receiver, the mobile terminal can accuratelydetermine its position using both cellular-based pseudosatellite signalsand available GPS signals.

[0036] It is another object of the present invention to provide aposition location system that includes a base station in a cellularnetwork that is capable of generating and transmitting GPSpseudosatellite signals independent of receiving GPS signals.

[0037] It is a further object of the present invention to provide aposition location system that includes a mobile terminal capable ofreceiving a GPS pseudosatellite signal from a base station of a cellularnetwork, and processing that signal as a substitute for a missing GPSsatellite signal, and in combination with available GPS satellitesignals, to determine position.

[0038] It is another object of the present invention to provide aposition location system that efficiently makes use of both a positionlocation scheme using the cellular network infrastructure and a GPSlocation system when the requisite number of GPS satellites are not inclear view of the mobile terminal containing the GPS receiver.

[0039] It is a further objective of the present invention to provide aposition location system that makes use of information from both the GPSand the cellular network infrastructure to provide improved accuracy andreliability than could be achieved by either system working alone.

[0040] It is yet a further object of the present invention to provide aposition location system that makes use of a position location schemeusing the cellular network infrastructure and a GPS location system asappropriate to minimize power consumption in the terminal.

[0041] It is a further object of the present invention to provide aposition location system that efficiently switches to a positionlocation scheme based on a combination of using the cellular networkinfrastructure and the available GPS satellite signals when therequisite number of GPS satellites are not in clear view of the mobileterminal containing the GPS receiver.

[0042] It is yet another object of the present invention to provide aposition location system that converts from a position location schemeusing the cellular network infrastructure to GPS when the requisitenumber of GPS satellites are in clear view of the mobile terminalcontaining the GPS receiver.

[0043] It is yet a further object of the present invention to provide aposition location system that utilizes GPS technology to reduce theproblems associated with the position location scheme that uses thecellular network infrastructure.

[0044] It is another object of the present invention to provide aposition location system that reduces the effects of radio multi-pathpropagation and clock drift associated with position location schemesusing cellular network infrastructure by utilizing GPS technology.

[0045] It is yet a further object of the present invention to provide aposition location system that efficiently utilizes cellular signals of aCDMA or TDMA system to augment the position determination.

[0046] In order to achieve these and other objects, the presentinvention provides a position location system that incorporatesparticular aspects of the cellular network with GPS. For one, in theposition location system of the present invention, the cellular networkis utilized to collect DGPS error correction information, and forward itto the mobile terminal over the established cellular network in thecellular band. The mobile terminal includes a DGPS processor thatprocesses the information, along with pseudoranges received from a GPSreceiver, in order to calculate a more precise position than thatobtained from GPS standing alone. In the alternative, the DGPS processoris connected to a communications network which is also connected to thebase station, and receives DGPS error correction data, along with thepseudoranges from the GPS receiver, over the cellular network (from themobile). The DGPS processor uses this information to correct thepseudoranges to obtain more accurate ranges.

[0047] Second, in the position location system of the present invention,when the requisite number of GPS satellites are not in view of the GPSreceiver, the system utilizes a GPS pseudosatellite signal that isgenerated by one or more base stations of the cellular networkindependent of the GPS, that is, independent of having to receive GPSsignals at the base station. The base stations are modified to generateand broadcast such pseudosatellite signals, and the pseudosatellitesignal is received and processed by the mobile terminal as a substitutefor an actual GPS satellite signal. Processing this information alongwith the satellite signal information from the GPS satellites that arein clear view, the mobile terminal is able to determine its positionmore accurately.

[0048] Third, in the position location system of the present invention,when the requisite number of GPS satellites are not in clear view of theGPS receiver, the system switches from relying on the GPS portion of thesystem to utilizing cellular network infrastructure to determinelocation. This can be done, for example, by using either the TOA or TDOAmethods for determining location in a cellular network portion of thesystem. Furthermore, when the mobile terminal is moved to a locationwhere the requisite number of satellites are again in clear view of theGPS receiver, the system efficiently switches back to using the GPSportion of the system to determine location. An alternative is to use acombination of GPS satellite signals and cellular signals from basestations to calculate position.

[0049] Fourth, the cellular signals already transmitted, for example, ina CDMA or TDMA (Time Division Multiple Access) system may be used as areplacement of a missing GPS signal or to augment and improve GPSmeasurements. Using either the CDMA or TDMA system, a round trip delayis calculated with respect to a base station, from which the radius ofthe terminal from the base station is calculated. Further calibrationcan be achieved by calculating a timing offset correction, to achieve amore accurate radius measurement.

[0050] Other objects, features, and advantages of the present inventionwill become apparent from reading the following Detailed Description inconjunction with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0051]FIG. 1 illustrates a conventional cellular network area dividedinto a plurality of cells.

[0052]FIG. 2 illustrates the major components of a conventional cellularnetwork scheme.

[0053]FIG. 3 illustrates the concept of determining position of a mobileterminal based on Time of Arrival measurements in a cellular networksystem.

[0054]FIG. 4 is a block diagram of a mobile telecommunications switchingoffice (MTSO) and base station for implementing a first aspect of theposition location system of the present invention.

[0055]FIG. 4A is a block diagram of a communications network and basestation for implementing a first aspect of the position location systemof the present invention.

[0056]FIG. 5 is a block diagram of a mobile terminal for carrying outthe first aspect of the position location system of the presentinvention.

[0057]FIG. 6 is a block diagram of an alternative embodiment of the MTSOand base station for implementing the first aspect of the positionlocation system of the present invention.

[0058]FIG. 6A is a block diagram of an alternative embodiment of acommunications network and base station for implementing a first aspectof the position location system of the present invention.

[0059]FIG. 7 is a block diagram of an alternative embodiment of themobile terminal for carrying out the first aspect of the positionlocation system of the present invention.

[0060]FIG. 8 is a block diagram of a base station for implementing asecond aspect of the position location system of the present invention.

[0061]FIG. 9 is a block diagram of a mobile terminal for carrying outthe second aspect of the position location system of the presentinvention.

[0062]FIG. 10 is a block diagram of a mobile terminal and base stationsaccording to a third aspect of the position location system of thepresent invention.

[0063]FIG. 11 is a flowchart depicting operation of the positionlocation system according to the third aspect of the present invention.

[0064]FIG. 12 is a flowchart depicting operation of the positionlocation system according to an alternative of the third aspect of thepresent invention.

[0065]FIG. 13 is a flowchart depicting operation of the positionlocation system according to a further alternative of the third aspectof the present invention.

[0066]FIG. 14 is a block diagram of a base station for implementing afourth aspect of the position location system of the present invention.

[0067]FIG. 15 is a flowchart depicting carrying out a correctioncalculation according to the fourth aspect of the position locationsystem of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0068] The various aspects of a position location system according tothe present invention are described below.

[0069] First Aspect

[0070] Referring to FIGS. 4 and 5, a first aspect of a position locationsystem of the present invention will be described. Broadly, FIG. 4 showsa source for DGPS error correction data 400, a cellular mobiletelecommunications switching unit 410, and a base station 440. FIG. 5shows a mobile terminal 500, which is typically at a remote locationrelative to the cellular mobile telecommunications switching unit and inthe transmitting vicinity of the base station 440. The transmittingvicinity is the area over which the base station broadcasts its signals.In general, this aspect of the invention involves the use of thecellular network to transmit DGPS error correction data to the mobileterminal, where it is used to perform corrections on pseudorange dataalso received at the mobile terminal.

[0071] First, in FIG. 4, the source 400 is responsible for providingDGPS error correction data (i.e., differential error correction data).Numerous alternatives for source 400 exist to provide such information,including using Government sources, commercial operators, or thecellular operator. For example, the source 400 may be a Governmentsource, such as the Coast Guard, which broadcasts DGPS error correctiondata as radio signals from reference stations that it has established.Alternatively, a commercial supplier may be used to supply the DGPSerror correction data. Two examples of such commercial suppliers areDifferential Correction, Inc. (DCI) of California, and Omnistar, Inc. ofTexas. In particular, DCI currently uses FM radio stations to broadcastthe correction information while Omnistar uses a geostationary satelliteto broadcast the correction information.

[0072] A third alternative is that the cellular provider set up its ownreference stations that calculate the pseudorange corrections for eachvisible satellite and broadcast them over the cellular network. In thatcase, the reference stations may be part of the base stations in thecellular network system.

[0073] For any particular application, depending on which alternative isselected for use as the source 400, the source will contain anappropriate receiver, such as a satellite receiver, an FM receiver, abeacon receiver, etc. For example, if Omnistar were used as the source,then a satellite receiver would be necessary. It should be noted thatthe signal received will be in the satellite frequency band. The typicaloverall satellite frequency band includes approximately 1200-1600 MHzand 3500-4300 MHz. The Omnistar system, for example, uses the 1551.489,1554.497 and 1556.825 MHz frequencies for its coverage of the UnitedStates.

[0074] Furthermore, the present invention contemplates a system that hastwo or more of the above-noted sources available, and obtaining theinformation from one or more of those sources as desired. For example,the system may have the ability to receive DGPS error correctioninformation from both a commercial supplier and reference stations setup by the cellular provider. In that circumstance, the source 400 wouldinclude circuitry that would decide which source to utilize, based on,for example, availability of each source.

[0075] In addition to receiving the differential error correction data,the source 400 will generally also convert the data into a standard DGPSsignal, such as, for example, as defined by RTCM SC-104 (Radio TechnicalCommission for Maritime Services, Special Committee-104), which hasdeveloped international standards for digital messaging. The DGPS signalis forwarded over a data link 405 to the cellular mobiletelecommunications switching unit 410, which is here a modified versionof a mobile telecommunications switching office (MTSO) known in the art.Data link 405 may be any type known in the art and compatible for usewith the MTSO 410.

[0076] Furthermore, relevant to this aspect of the invention, the MTSO410 includes a processing unit 415, a central unit 420 that is capableof receiving data and messages from other sources, a multiplexer 425,and a switching unit 430. Processing unit 415 is responsible forconverting the DGPS signal into a proper format for further transmissionover the cellular network. For example, processing unit 415 may convertthe received signal into a short message by using a short messageservice (SMS) as defined in the Global System for Mobile Communication(GSM) standard. GSM represents a mobile cellular system as defined by aset of operating standards, as introduced by the European body ETSI. Forthe purposes of understanding the present invention, a short message isessentially a data packet containing the DGPS signal.

[0077] Central unit 420 contains other data and message sources that aresupplying information that must be forwarded by the MTSO 410. Suchinformation would include other short messages intended for transmissionto the same base station, voice data, data traffic for users havingmodems at the mobile terminal, and the like. This other information inthe central unit 420 is combined with the short message containing thedifferential error correction data using multiplexer 425 to create acombined signal.

[0078] Thereafter, the combined signal is transmitted to a switchingunit 430 of the MTSO 410 which is responsible for switching the data toan appropriate data link 435 for forwarding to one of numerous cellularnetwork base stations in the cellular system. Here, the intended basestation for receiving the combined signal is represented by referencenumeral 440. Base station 440 includes a base station modulator 445 thatmodulates the signal, and then transmits it through a radio interface450 and a base station antenna 455. It should be noted that thetransmission of the signal by the base station will be in the cellularfrequency band. The typical overall cellular frequency band includesapproximately 800-900 MHz and 1850-1990 MHz.

[0079] An analogous FIG. 4A shows the preferred arrangement. In thisarrangement the source of DGPS data 400 is connected to a communicationsnetwork 460 by a data link 406. In this view the source of DGPS data maybe, for example, a workstation or server attached to the Internet andproviding DGPS data for many base stations in one or more mobilenetworks. Although this server provides a logically separate function,it may be combined with, or physically located in conjunction with, apart of the communications network (for example such as an MTSO). Itshould be noted that the communications network may simply be an MTSO(as shown in FIG. 4), include an MTSO or a plurality of MTSOs with othercomponents, or may itself not include any MTSOs. The server may also beoperated by a third party, separate from the mobile network operator,and located remotely from the communications network components. Thebase station 440 is also connected to the communications network throughdata link 436. The communications network interconnects the source ofDGPS data and the base station and provides the similar functions to theMTSO shown in FIG. 4 of receiving messages from the DGPS source andcombining these together with other data and messages destined for thebase station for transmission to the mobile terminals served by the basestation. Such a communications network, for example, is provided by theInternet and the associated Internet protocols (IP) for addressing,formatting, sending and receiving messages to devices attached to thenetwork.

[0080] The signal containing the short message with the DGPS errorcorrection data is transmitted as a radio frequency signal in thecellular band to the mobile terminal 500, which is shown in FIG. 5.Mobile terminal 500 generally includes a cellular antenna 505, a radiointerface 510, a digital signal processor (DSP) 512, a central processor515, a DGPS processor 520, a control unit 525, a speaker 530, and a GPSreceiver 550 having a GPS antenna 555. Mobile terminal 500 receives theradio frequency signal via the cellular antenna 505, and forwards it tothe radio interface 510. Radio interface 510 converts that signal intoan intermediate frequency (IF) signal that is compatible with the DSP512, and forwards the signal to the DSP 512. The DSP 512 demodulates thesignal into a data stream and forwards that data stream to the centralprocessor 515. Central processor 515 is responsible for determining thecontents of the data stream and forwarding the appropriate portions ofthe data stream to their intended destinations. For example, a typicaldata stream may contain short messages, one of which is the DGPS errorcorrection data, control data, voice data, and other information.

[0081] Regarding the short message carrying DGPS error correction data,the central processor 515 utilizes the protocol of the short message,which use of protocol is known in the art, to extract the errorcorrection data out of the signal and forward it to the DGPS processor520. In addition, for illustrative purposes, the central processor 515forwards control data to the control unit 525, while voice data may beprocessed and forwarded to the speaker 530. FIG. 5 does not depict allof the various destinations for all types of data encountered by thecentral processor 515.

[0082] In addition, the GPS receiver 550 receives satellite signals bythe GPS antenna 555 from the GPS satellites that are in view, andcalculates the pseudoranges between the mobile terminal and each of theGPS satellites. GPS receiver 550 forwards these pseudoranges to the DGPSprocessor 520.

[0083] DGPS processor 520 utilizes the DGPS error correction data in aconventional manner to correct the calculated pseudoranges. Thecorrected ranges are forwarded back to the GPS receiver 550, which thenutilizes the information to calculate a more accurate position of themobile terminal 500 than could be achieved by using GPS standing alone.

[0084] It should be noted that the cellular antenna 505 and the GPSantenna 555 may be formed as a single antenna that is capable ofreceiving both types of signals, and may use, for example, an amplifierthat responds to both cellular and satellite bands.

[0085] An alternative structure for carrying out the correction of thepseudoranges is shown by FIGS. 6, 6A and 7. Much of the structureremains similar as in the embodiment shown in FIGS. 4, 4A and 5, forwhich analogous reference numerals have been used. However, in thisembodiment, the mobile terminal 700 has no DGPS processor. Rather, aDGPS processor 675 is connected to a source of DGPS data 600 by a datalink 605, and is connected by another data link 612 to a processing unit615 (which may be a multitude of processors) in a MTSO 610.

[0086] The remaining elements are a central unit 620, a multiplexer 625,and a switching unit 630 in the MTSO 610, a data link 635, and a basestation 640 that includes a base station modulator 645, a radiointerface 650 and a base station antenna 655.

[0087] An analogous FIG. 6A shows the preferred arrangement. In thisarrangement the source of DGPS data 600 is connected to a communicationsnetwork 660 by a data link 606. In this view the source of DGPS data maybe, for example, a workstation or server attached to the Internet andproviding DGPS data to one or more base stations in one or more mobilenetworks. Similarly the DGPS processor 675 is also attached to thecommunications network through data link 614. In this view the DGPSprocessor may also be, for example, a server attached to the Internetand providing DGPS processing services for one or more base stations andtheir associated mobile terminals in one or more mobile networks. Thebase station 640 is also connected to the communications network throughdata link 636. The communications network interconnects the source ofDGPS data, the DGPS processor and the base station and provides thesimilar functions to the MTSO shown in FIG. 6 of receiving messages fromthe DGPS source and combining these together with other data andmessages destined for the base station for transmission to the mobileterminals served by the base station. Similarly the communicationsnetwork provides the function of transporting messages reporting GPSpseudoranges received by the base station from the mobile terminals tothe DGPS processor. Such a communications network, for example, isprovided by the Internet and the associated Internet protocols (IP) foraddressing, formatting, sending and receiving messages to devicesattached to the network.

[0088] Furthermore, the mobile terminal 700 contains a cellular antenna705, a radio interface 710, a digital signal processor (DSP) 712, acentral processor 715, a control unit 725, a speaker 730, and a GPSreceiver 750 having a GPS antenna 755.

[0089] In this embodiment, the GPS receiver 750 receives satellitesignals by the GPS antenna 755 from the GPS satellites that are in itsview and calculates the pseudoranges between the mobile terminal 700 andeach of the GPS satellites. The results of this determination, i.e., thecalculated pseudoranges, are then forwarded through the system to theDGPS processor 675 as described below. It should be noted that this maybe done automatically in a continuous fashion, i.e., each time the GPSreceiver 750 calculates pseudoranges it also forwards the pseudorangesto the DGPS processor 675. Alternatively, it may be done upon a specificrequest to the GPS receiver 750.

[0090] Forwarding of the calculated pseudoranges to the DGPS processor675 may be carried out by forwarding the calculated pseudoranges to thecentral processor 715, which multiplexes the calculated pseudorangesinto a data stream that is forwarded to the DSP 712. The DSP 712modulates the data stream into an IF signal and forwards the signal tothe radio interface 710. The radio interface 710 converts the IF signalinto a radio frequency signal having a frequency that is in the cellularband. The cellular signal is transmitted to be received at the basestation 640. Base station 640 demodulates the signal, and forwards thedemodulated signal message through the data links to the DGPS processor675. If the signal message passes through data link 635 to the MTSO 610,the MTSO extracts the calculated pseudorange messages, for example, byusing the processing unit 615, and forwards them over the data link 612to the DGPS processor 675. If the signal message passes through datalink 636 to a communications network, it will be received by the DGPSprocessor through data link 614 where the relevant pseudorange messagesmay be extracted.

[0091] In addition, the source 600 provides a signal containing the DGPSerror correction data to the DGPS processor 675. Therefore, the DGPSprocessor 675 will have the pseudoranges as calculated by the GPSreceiver 750, along with the necessary error correction data from thesource 600. DGPS processor 675 performs the required corrections on thecalculated pseudoranges and transmits the corrected ranges back throughthe system to the GPS receiver 750. GPS receiver 750 then calculates amore accurate position of the mobile terminal 700 using these correctedpseudoranges.

[0092] In the alternative, DGPS processor 675 could calculate thecorrected position of the mobile and forward the position to the mobilefor display, or to the network service or other service requesting theposition. The requesting service may be a process of the cellularnetwork itself, or it may be a server operated by a third partyconnected to the communications network.

[0093] It should be noted that in either of the embodiments shown inFIGS. 4 (4A), 5 and FIGS. 6 (6A), 7, one or more of the elements may beintegrally formed with other elements, i.e., be formed as part of asingle unit. For example, the DGPS processor may be integral with thecentral processor or the GPS receiver of the mobile terminal. Inaddition, while not described in detail, the DGPS processor mayalternatively be located at the base station or other locations andconnected to the base station through the communications network. Inaddition, the first aspect of the position location system of thepresent invention may be used with either a local area DGPS (LADGPS)system or a wide area DGPS (WADGPS) system.

[0094] This aspect of the invention provides numerous advantages overthe conventional systems. For one, there is no need for a separatebeacon receiver at each mobile terminal for receiving the DGPS errorcorrection information from the source. That is, in the conventionalDGPS approach, each individual mobile terminal containing a DGPSreceiver/processor needs a beacon receiver for receiving the DGPSsignal. By way of the present invention, the need for a beacon receiverat each mobile terminal is eliminated. Rather, the mobile terminal needonly have a DGPS processor to process the DGPS signal and correct thepseudoranges calculated by the GPS receiver. This means a reduction inthe amount of structure present in the mobile terminal, thereby allowingfor a smaller size and lower cost.

[0095] Furthermore, in the embodiment where the DGPS processor isconnected to the base station through the communications network, thereis also no need for a separate DGPS processor at each mobile terminal.This reduces the number of DGPS processors necessary, since there is noneed to have a separate processor in each mobile terminal. This allowsfor a further reduction in size and cost of the mobile terminal withoutsacrificing the accuracy of position determinations made possible usingDGPS. This configuration also reduces the message traffic between thenetwork and the terminal by allowing the position location determined bythe DGPS processor to be sent directly to the network service requestingthe information.

[0096] In addition, the present invention utilizes the frequency bandalready dedicated for cellular use for performing additional functions,here the transmission of DGPS error correction data. Therefore, moreefficient use of the cellular band is made with this invention.Moreover, the cellular network provides an efficient means for havingthe DGPS error correction information transmitted to the mobileterminal. That is, in certain instances, the mobile terminal may belocated in an area where reception of the DGPS signal from a specifiedreference station is not possible. For example, the mobile terminal maynot be within the proper range of the reference station. The cellularnetwork is generally available in such circumstances, and it can providethe necessary DGPS error correction information. Therefore, the userwill not lose the accuracy attributable to DGPS.

[0097] Second Aspect

[0098] Referring now to FIGS. 8 and 9, a second aspect of the positionlocation system of the present invention will be described. This aspectaddresses the problem described above of a GPS receiver not having therequisite number of GPS satellites in clear view. For example, in atypical situation, four GPS satellites must be in clear view of the GPSreceiver in order for the GPS receiver to gain an accurate fix on itslocation. Of course, the requisite number of GPS satellites that must bein view may be less than four when the GPS receiver has one or morecomponents of its location already known. It is assumed for the purposesof illustration of this embodiment that four GPS satellites are needed,but only three of those satellites are in clear view. This may occur,for example, when the GPS receiver is in a city setting, such as in anurban canyon, i.e., in the shadow of a group of tall buildings, orindoors. Of course, there are numerous other reasons why the GPSreceiver may not be receiving signals from the requisite number of GPSsatellites, which are not listed here.

[0099] Broadly, in the present invention, a base station of the cellularnetwork provides a pseudosatellite signal that may be used by the GPSreceiver as a replacement for a GPS satellite that is not in clear view.Furthermore, the base station provides the pseudosatellite signalswithin the cellular band, i.e., over the cellular network. In order tocarry out this aspect of the invention, the base station needs to bemodified, along with the mobile terminal, as described below.

[0100] Referring to FIG. 8, a base station 800 according to this aspectof the present invention is shown. Base station 800 includes a timestandard 805 and a GPS processing unit 810. The time standard may be anindependent reference unit such as a commercially available cesium basedreference clock. Alternatively, the cellular network infrastructure,used to synchronize base station transmissions, may also be used toprovide a time reference. GPS processing unit 810 is programmed withlocation code and is responsible for calculating pseudo GPS datarepresenting the base station's location and other related information.In addition, a C/A code unit 820 provides the encoding scheme forsignals in the civilian band. It should be noted that the base station800 does not act as a conduit for forwarding GPS satellite informationin this aspect of the invention. Rather, the base station 800 produces apseudosatellite signal that is based on the programmed code in the GPSprocessing unit 810 and the encoding scheme of the C/A code unit 820.The calculated pseudo GPS data from the GPS processing unit 810 and theC/A code generated by the C/A code unit 820 are combined by a modulator825 to produce a pseudosatellite signal. Of course, other generators mayreplace the shown GPS processing unit 810, C/A code unit 820 andmodulator 825, as long as they are able to generate a GPSpseudosatellite signal. The pseudosatellite signal produced has the samecharacteristics as a normal GPS satellite signal.

[0101] Next, in order to place the signal into proper form fortransmission over the cellular band, a converter converts thepseudosatellite signal to an appropriate radio frequency (RF) signal fortransmission over the cellular band. One example of a converter isillustrated as an up converter 830, which is supplied with a localcarrier frequency, and a filter 835. In this embodiment, the filter 835has a 2 MHz bandwidth, with a center frequency equal to the localcarrier frequency.

[0102] In addition, the base station 800 may also be transmitting othersignals created by a cellular radio 845. These represent the typicalsignals that are transmitted to the mobile terminals to provide thetypical speech and signaling services associated with mobile phones. Ifboth signals need to be transmitted together, it is desirable to set thepseudosatellite signal at a level such that it does not interfere withthe remaining cellular signals that are to be transmitted. One possibleworking parameter is to have the pseudosatellite signal broadcast at alevel which is at least twenty decibels (dB) lower than the remainingcellular signals.

[0103] To carry out this level adjustment, the RF signal is passedthrough a level adjuster 840 which adjusts the amplitude level of the RFsignal such that it is at some predetermined level below that of theremaining cellular signals transmitted through the cellular radio 845.The level of the cellular band signals from the cellular radio 845 mayvary over time depending on the cellular system traffic or any transmitpower control process utilized by the cellular system. The leveladjuster 840 may thus change the level of the pseudosatellite signalover time (dynamically) in response to changes in cellular radiotransmissions in order to keep the pseudosatellite signal at the maximumlevel possible, yet not interfere with the cellular transmissions.

[0104] Thereafter, the signals from the cellular radio 845 and thepseudosatellite signal (i.e., the RF signal) are combined by a combiner850. The combined signal is adjusted by a power amplifier 855 and thentransmitted in the cellular band of the cellular network.

[0105] It should be understood that, at this point, the base station 800has created the pseudosatellite signal such that it can be broadcast,together with other cellular signals, in the cellular band of thecellular network to the mobile terminal. The pseudosatellite signal maybe decoded by the mobile terminal to determine the range to the basestation. The mobile terminal is described in connection with FIG. 9below.

[0106] Turning to FIG. 9, a mobile terminal 900 according to the secondaspect of the present invention is described in detail. Mobile terminal900 generally contains a cellular mobile portion 905 and a GPS receiverportion 910, and a control processing unit 940. Cellular mobile portion905 generally includes a cellular antenna 915, a low-noise amplifier920, a down converter 925, a filter 930, an IF section and base bandprocessor 935, and audio outputs 936. The GPS receiver portion 910generally includes a GPS receiver antenna 945, a low-noise amplifier950, a first down converter 955, a first filter 960, a second downconverter 965, a second filter 970, an analog-to-digital (A/D) converter975, and a digital signal processor (DSP) 980. In addition, the mobileterminal 900 contains an automatic gain controller or amplitudecontroller 990 and a switch 995. As readily understood by one ofordinary skill in the art, the particular components shown for thecellular mobile portion and the GPS receiver portion are not necessarilyexhaustive of the components contained therein, which any othercomponents are generally known in the art. Furthermore, the presentinvention contemplates substitutes of the components shown that arecapable of carrying out substantially the same functions.

[0107] It should be noted that the cellular antenna 915 and the GPSantenna 945 may be formed as a single antenna that is capable ofreceiving both types of signals. This alternative has the advantage ofcreating a much more compact mobile terminal. If the single antenna isused, then the low noise amplifiers 920 and 950 may also be combinedinto a single unit, with response designed for both cellular and GPSbands, and with outputs to connect to down converters 925 and 955.

[0108] The cellular signal broadcast from the base station 800, whichcontains both the pseudosatellite signal and other cellular signals, isreceived at the mobile terminal 900 by the cellular radio antenna 915,and is then processed through the low-noise amplifier 920, the downconverter 925, and the filter 930. These last three elements work toconvert the received signal, which has a radio frequency in the cellularband, into a signal having a predetermined intermediate frequency (IF).Of course, any suitable converter may replace the shown low-noiseamplifier 920, down converter 925, and filter 930. The IF signal isforwarded to the IF section and base band processor 935, and dependingon its contents, is forwarded to the necessary destination in the mobileterminal 900. For example, voice data would be forwarded to audiooutputs 936, and control data is forwarded to the control processingunit 940. In addition, the IF signal is also forwarded to the automaticgain controller 990 and thereafter to the switch 995, at input A. Theautomatic gain controller 990 adjusts the amplitude of thepseudosatellite signal to correspond to the GPS satellite signals, whichare described below.

[0109] It should be noted that the filter 930 must be constructed to becapable of handling pseudosatellite signals, which are typically widerthan conventional cellular signals. Based on the present scheme ofsatellite signals transmitting at frequencies described above—e.g., C/Acode with 1.023 MHz rate—the filter 930 should have the requisite width.For example, a width of at least approximately 2 MHz may be used. Inthat regard, it is most practical if the cellular system have abandwidth that is similar to the pseudosatellite signal bandwidth. Thetypical bandwidth of IS-95 CDMA cellular signals is approximately 1.25MHz. Therefore, a GPS pseudosatellite signal that is approximately 2 MHzwide could underlay the center of one IS-95 channel, which would meanthat it would also overlap two adjacent channels. Other options fortransmission over the IS-95 channels could be used. The IF section of935 will typically include a channel selection filter to separate thedesired cellular channel from the other, overlapped, cellular channels.

[0110] Moreover, an alternative to the filter 930 would be to have twofilters. In such a scenario, one filter handles the pseudosatellitesignals and forwards them to automatic gain controller 990. The otherfilter handles the cellular signals and forwards them to the IF sectionand base band processor 935. With this dual filter arrangement, thechannel selection is performed together by the cellular section offilter 930 and the channel selection filter section of 935.

[0111] Referring now to the GPS receiver portion 910, it receivessatellite signals by the GPS antenna 945 in the satellite frequency bandfrom the GPS satellites (not shown) that are in clear view. The GPSreceiver portion 910 converts those signals to correspond to the IFsignal representing the pseudosatellite signal; that is, having the samefrequency and substantially the same amplitude. That conversion may becarried out by routing the GPS satellite signals through the low-noiseamplifier 950, the first down converter 955, and the first filter 960.The IF signal of the GPS satellite signals is sent to the switch 995, atinput B.

[0112] The control processing unit 940 is generally responsible forcontrolling the operation of the switch 995. The control processing unit940 can be programmed such that it normally has the switch set to inputB, but when it determines that it is not receiving the requisite numberof GPS signals, it toggles the switch to input A to accept thepseudosatellite signal. Thus, the control processing unit 940 utilizesas many GPS satellites that are in view, and toggles to input A to alsoreceive a pseudosatellite signal when the requisite number of GPSsatellites are not in view of the mobile terminal. Those of ordinaryskill in the art will recognize that there are numerous alternatives tothe structure shown for switching between the pseudosatellite signalsand the GPS satellite signals.

[0113] Therefore, the switch 995 chooses the signal at either input A orinput B. The selected signal is then processed by another converterhere, the second down converter 965 and the second filter 970—in orderto convert the signal into a baseband signal. Thereafter, the signal isconverted from an analog signal to a digital signal by the A/D converter975. Next, the DSP 980 processes the signal to produce a data stream.The control processing unit 940 receives the data stream from the DSP980 and derives the position of the mobile terminal 900 in a mannerknown in the art, including calculating pseudoranges that are used todetermine the position. It should be noted that the function ofcalculating position may be carried out by a separate processing unitthat is separate from the control processing unit 940.

[0114] Thus, the control processing unit 940 is configured to utilizethe GPS signals from GPS satellites that are in view, and when therequisite number of GPS satellites are not in view, to also utilize one(or more) pseudosatellite signals from base stations to substitute forthe missing GPS satellite signal. That is, a combination of GPSsatellite signals and the pseudosatellite signals are utilized tocalculate position of the terminal. This approach has the advantage ofutilizing GPS, which provides the most reliable position data, to thefullest extent possible, and only rely on the pseudosatellite signals asnecessary when GPS alone does not provide the required information.

[0115] In the above-described embodiment, the base station may becontrolled to broadcast the pseudosatellite signal as a continuous wavesignal, i.e., in a continuous manner at all times, or may be controlledto broadcast the signal in a burst mode—i.e., broadcast, for example,twenty percent of the time. If the pseudosatellite signals are broadcastin bursts, the mobile portion 905 must synchronize its reception withthe bursts which it may do through knowledge of the burst timing. In anyevent, the mobile terminal 900 receives this pseudosatellite informationwhenever it is broadcast by base station 800. Therefore, it can be seenthat the broadcast of the pseudosatellite signal by the base station800, and the receipt of the pseudosatellite signal by the cellularmobile portion 905 in the mobile terminal 900, is carried out withoutregard to whether GPS receiver portion 910 actually has four GPSsatellites in view.

[0116] However, when the GPS receiver portion 910 has four GPSsatellites in view, the pseudosatellite signal is not needed. If nothingelse is done, this signal is disregarded due to the switch 995. However,if desired, the control processing unit 940 may be programmed to utilizethe pseudosatellite signal even when the requisite number of GPS signalsare available. In such a case, the processor 985 will have available toit five or more pseudoranges (four or more from the GPS satellites andone or more from the base stations). This exceeds the number of signalsnecessary to carry out a position calculation, but does not negativelyaffect the position calculation made, and indeed, can improve theaccuracy of the position calculation in such an over-determined system.

[0117] It should further be noted that in this embodiment of theinvention, the base station 800 need not have any GPS satellite signalreceiving capabilities itself. That is, the base station produces apseudosatellite signal independent of receiving and utilizing GPSsignals, and therefore it need not have the structure necessary forreceiving actual GPS satellite signals. Rather, the base station 800simply produces a pseudosatellite signal based on a code programmedtherein and a particular time reference, which need not be derived fromGPS.

[0118] This aspect of the invention provides an efficient scheme forproducing a pseudosatellite signal, and forwarding that signal to themobile terminal. The present invention accomplishes this by using anestablished network—the cellular network—to carry out these functions.By adding very little additional structure to already existing basestations, the need to build dedicated reference stations at strategiclocations is obviated. Furthermore, the present invention provides formore efficient use of the cellular band by utilizing it to sendadditional types of information.

[0119] Moreover, the coverage provided by the cellular network generallyincludes the strategic locations where dedicated reference stationswould otherwise be placed, including at airports and in city settings.Furthermore, the cellular network has the advantage of providingcoverage where the available number of GPS satellite signals is likelyto be insufficient, including in city settings such as in urban canyonsor indoors.

[0120] In addition, the present invention overcomes the problem thatwith dedicated reference stations broadcasting pseudosatellite signals,the pseudosatellite signal is stronger than actual GPS satellite signalsand drowns out the GPS satellite signals. In the present invention, thepseudosatellite signal produced and broadcast by the base station willnot interfere in any way with the actual GPS satellite signals, andtherefore will not drown out the GPS satellite signals. In fact, the twoare broadcast in completely different frequency bands. That is, the GPSsatellite signals are broadcast in the satellite band, while the basestation broadcasts the pseudosatellite signal in the cellular band.

[0121] Furthermore, when the alternative employing the switch 995 isused, then receipt of the pseudosatellite signal by the controlprocessing unit 940 may be completely prevented when four GPS satellitesare in view of the GPS receiver portion. Again, this is an improvementover the conventional system as it enables the local control to selectbetween the pseudosatellite signals and the GPS satellite signals thatare available.

[0122] As referred to above, the second aspect of the invention may alsobe expanded to where more than one base station transmitspseudosatellite signals, and therefore, the position of the mobileterminal 900 may be determined even when two (or more) GPS satellitesare not in view. For example, two base stations may transmitpseudosatellite signals, such that the GPS receiver portion 910 mayaccurately determine its location when only two GPS satellites are inits view. Indeed, any combination of base stations and satellites couldbe used. When no GPS satellites are in view, then four base stationscould be used to calculate position. Of course, it may also be desirableto use as many GPS satellites that are in view, along with all theavailable pseudosatellite signals, to obtain an over-determined resultthat may be more accurate.

[0123] Third Aspect

[0124] Referring now to FIGS. 10-13, a third aspect of the presentinvention will be described. As with the second aspect of the invention,this aspect also addresses the situation where the requisite number ofGPS satellites are not in view of a GPS receiver. Broadly, in thisaspect of the invention, the mobile terminal has the capabilities ofcalculating its position using GPS, and also using the cellular networkinfrastructure, such as by the TOA or TDOA methods of determininglocation. The mobile terminal will generally use the GPS structure tocalculate position. However, when the requisite number of GPS satellitesare not in view, then it will switch to calculating position usingeither only the cellular network infrastructure, or a combination of theGPS satellite signals available and the cellular network infrastructure.When the requisite number of GPS satellites return into view, then themobile terminal switches back to relying exclusively on GPS.

[0125] First, in FIG. 10, a mobile terminal 1000 includes a GPS receiverportion 1005, a mobile cellular portion 1010, and a central processor1015. Of course, the central processor 1015 may alternatively be formedintegral with either the GPS receiver portion 1005 or the mobilecellular portion 1010, or a single processor can be used that performsthe functions of all three components. GPS receiver portion 1005includes a GPS processor 1020 for calculating position, while the mobilecellular portion 1010 contains a cellular position processor 1025 thatcomputes position using the cellular network infrastructure. Finally, asshown in FIG. 10, base stations 1030, 1035, and 1040 are part of thecellular network, and for purposes of explanation, they represent thebase stations whose transmitting vicinity includes the location of themobile terminal 1000.

[0126] Operation of the mobile terminal 1000 for determining position isdescribed in connection with the flowchart in FIG. 11. At block 1100,GPS receiver portion 1010 obtains a first fix on the location of themobile terminal, if necessary. Next, at block 1105, the first fixlocation and the locations of three (or more) nearby base stations aretransmitted to cellular position processor 1025. In this case, thenearby base stations are base stations 1030, 1035, and 1040. Cellularposition processor 1025 utilizes this information, along with thelocation of mobile terminal 1000 as determined in step 1100, todetermine the expected time difference of arrival of periodic signalsfrom base stations 1030, 1035, and 1040. For the purposes of thisexplanation, it is assumed that periodic signals are available in thecellular network, and the mobile terminal uses the TDOA process,described earlier, to determine its position from the cellular networksignals. However, the cellular position processor could alternativelyuse a TOA technique.

[0127] In particular, in order to calculate the TDOA of the periodicsignals from each base station 1030, 1035, 1040, the cellular positionprocessor 1025 calculates the distance between the mobile terminal 1000and each of the base stations 1030, 1035, and 1040, using the locationmeasurement for each item. Using the known distances and the speed ofpropagation of the radio signals, the cellular position processor 1025calculates the expected time difference of arrival of the signals fromeach base station pair. In a completely synchronized system, the time oftransmission of the periodic signals from each base station is the same,or at some specified time offset (which can be subtracted out). Bycomparing the expected TDOA with the measured TDOA, the cellularposition processor 1025 can determine the time offset of the signalsfrom each base station, and use these to correct later TDOA measurementsfor these base stations. In addition, the range, or distance to the basestation, may be calculated from the amount of time that it took for theperiodic signal to travel from the base station to the mobile terminal,i.e., the propagation delay of the signal.

[0128] Therefore, knowledge of the position of mobile terminal 1000—ascalculated by the GPS receiver portion 1005—helps to determine the timeoffset of transmission of the periodic signals from the base stations,which may be unknown in cellular systems (except, for example, some CDMAsystems implementing the IS-95 standard). This time offset oftransmission for each base station pair is stored in cellular positionprocessor 1025. Alternatively, the time offset of transmission may bestored in central processor 1015.

[0129] Thereafter, as long as the requisite number of GPS satellites arein view of GPS receiver portion 1005, the GPS receiver portion continuesto calculate position in this manner. Thus, in block 1110, the GPSreceiver portion 1005 determines whether the requisite number of GPSsatellites are in view. If the requisite number of GPS satellites are inview (i.e., the answer to the inquiry is “yes”), then at block 1115, theGPS receiver portion 1005 again calculates (or simply updates) theposition of the mobile terminal using GPS. This process is generallyrepeated every second or few seconds. Of course, the time betweensuccessive calculations can be set to any desired amount. In addition,at block 1115, the time offset of transmission of the periodic signalsfrom each base station may be recalculated and updated, and stored inthe cellular position processor 1025 or the central processor 1015.Then, the next step will be at block 1110, where the same inquiry as tothe requisite number of GPS satellites being in view is made.

[0130] However, if the answer to the inquiry at block 1110 is “no,” thatis, the number of visible GPS satellites has dropped below the requisitelevel (usually four), then the position determination for the mobileterminal 1000 is switched over to the cellular position processor 1025.This is shown at block 1120. The switching over may be carried out bythe central processor 1015. In addition, the information of the lastknown position of the mobile terminal 1000 and each of the base stations1030, 1035, and 1040, along with the time offset of transmission of theperiodic signal from the base stations, is made available to cellularposition processor 1025 if it does not already have the information. Thecellular position processor 1025 calculates position using the periodicsignals from the base stations, as described below.

[0131] Turning to operation of the cellular position processor 1025,indicated at block 1125, the processor 1025 will measure the TDOA forthe signals from the base stations. However, because the mobile terminalhas likely since moved, the measured TDOA will differ from the previousvalues after correction for the transmission time offsets. The processor1025 may then calculate the new position using the new, corrected TDOAmeasurements as the intersection of the hyperbolic surfaces as discussedearlier.

[0132] In essence, at block 1125, the cellular position processor 1025uses the cellular network infrastructure to determine its location, forexample, by using either the TOA or TDOA methods for determininglocation, as discussed previously. Alternatively, any other method ofdetermining location based on the cellular network infrastructure can beused. In general, the point is that the cellular position processor 1025calculates position based on using cellular position signals, such asthe periodic signals, rather than using GPS satellite signals. That is,the cellular position signals do not contain GPS information and ratherare independent of GPS. Thus, this aspect is unlike the second aspectwhere the pseudosatellite signals generated and forwarded by the basestation are like GPS signals.

[0133] An alternative to switching over to using a method fordetermining position of the mobile terminal based exclusively on thecellular network infrastructure, such as the TOA or TDOA method, is touse a combination of the GPS satellite signals and the base stationperiodic signals to determine location. This is generally indicated bydotted block 1126 and the associated dotted lines. For example, forpurposes of explanation, it may be assumed that only three GPSsatellites are in view of the GPS receiver portion. The pseudorangedetermined for two of these satellites provides a distance measurementbetween each GPS satellite and the mobile terminal. The third satellitesignal provides a time reference used to calculate the range to theother two satellites. That is, the mobile terminal 1000 lies somewhereon a sphere around the GPS satellite, having a radius equal to thedistance therebetween. In addition, calculating the distance between themobile terminal and one base station, the method for which is describedabove, provides a third distance measurement. Therefore, the three GPSsatellites, along with the one base station, provides three sphereswhose intersection represents the location of the mobile terminal. Ofcourse, more than one base station may be used to ensure that anaccurate location determination is made.

[0134] In either case—using the method according to block 1125 or block1126—once the location determination is made, the next inquiry at block1130 will be the same as the inquiry made at block 1110, i.e., whetherthe requisite number of GPS satellites are in view of the GPS receiverportion. If the requisite number of GPS satellites are not in view, thenthe same process—in either block 1125 or block 1126—is performed todetermine position. This loop between blocks 1125 or 1126 and block 1130continues until the requisite number of GPS satellites come back intoview.

[0135] When enough GPS satellites are again in view of the GPS receiverportion, then the answer to the inquiry at block 1130 becomes “yes,” andthe system proceeds to block 1135. At block 1135, determination ofposition again becomes the responsibility of GPS receiver portion 1005.This switching back to using the GPS receiver portion for calculatingposition may be carried out by the central processor 1015. In addition,at this block, determinations of the location of the nearest basestations, and their locations and time offsets of transmission of theirperiodic signals, are all updated. In sum, the system is recalibratedjust as if it started out using GPS. Thereafter, the process is returnedto block 1105, and is repeated.

[0136] In addition, it should be noted that at block 1100, if an initiallocation fix cannot be made using the GPS receiver portion 1005 becausethe requisite number of GPS satellites are not in view, then the centralprocessor 1015 can request that the cellular position processor 1025provide an initial location of the mobile terminal using theconventional TDOA method of determining location using the cellularnetwork infrastructure, as described previously herein. However, oncethe requisite number of GPS satellites come into view of the GPSreceiver portion, then the central processor 1015 will cause the systemto switch over to determining location of the mobile terminal using theGPS receiver portion.

[0137] The GPS provides good accuracy of position when its signals areavailable to the mobile terminal. This may be more accurate than methodsusing exclusively signals provided by the cellular networkinfrastructure. Thus, it can be preferable to make use of the GPS fordetermining position, and to make use of a combination of the GPS andthe cellular infrastructure when the GPS is unavailable, or partlyunavailable, to the mobile terminal. It is, of course, preferable tomake use of as many signals from both systems as are available todevelop a more accurate result than could be obtained by working witheither system exclusively.

[0138] However, a competing interest is reducing power consumption. Asshown in FIG. 10, two receivers are present in the terminal. To savepower consumption (i.e., save battery power), it may be desirable torely on the cellular network infrastructure to calculate position.During this time, the GPS receiver would not be turned on. Only when thecellular network infrastructure does not provide the necessary signalsto determine position, or if a GPS recalibration was needed, would theGPS receiver be turned on and utilized to determine position. Thisalternative approach conserves battery power.

[0139] The alternative approach is shown in FIG. 12. At block 1200, afirst fix is calculated. Then, at block 1210, the GPS receiver is turnedoff, and at block 1220, position is calculated using the cellularnetwork infrastructure. At block 1230, a determination is made whetherthe requisite number of signals in the cellular network infrastructureare available for calculating position (e.g., three signals in the TDOAapproach). If so, then block 1220 is repeated. If not, then at block1240, the GPS receiver is turned on and the position is calculated usingGPS. Thereafter, the process is repeated starting at block 1210.Alternative inquiries at block 1230 include determining how long (intime) it has been since the last GPS update and/or how far (in space)the mobile terminal has moved since the last GPS update. If apredetermined amount of time has passed, such as two minutes, or themobile terminal has moved a predetermined distance, such as 100 meters,then a GPS update would be called for and the process proceeds to block1240. Of course, the rate at which GPS updates are needed will depend onthe conditions under which the mobile is operating, with less frequentupdates needed if the mobile terminal is in an area where the cellularnetwork infrastructure signals are known to be of good accuracy.

[0140] Another approach with the third aspect for determining positionis that shown in FIG. 13. At block 1300, a first fix is obtained. Atblock 1310, all the available GPS satellite signals are received by theGPS receiver portion. Then, at block 1320, all the available cellularpositioning signals are received. At block 1330, position is calculatedusing all of these signals. This is an overdetermined system, and theresults may be combined. This may be done by weighting each locationmeasurement in the average by the confidence, or expected error, in themeasurements. Such a technique is known as weighted average. Thistechnique may be used to combine together location measurements based onavailable GPS signals with measurements based on cellular infrastructuresignals with appropriate regard to the accuracy of each measurement. Theweighted average may be calculated according to the following generalexpression for an averaged position coordinate x

x=1/wΣx_(n)/σ_(n) ²

[0141] where x_(n) are the measurements, σ_(n) are the expected errors,or variances, in the measurements used to weight the average, andw=Σ1σ_(n) ² is the sum of the variances used to normalize the estimate.The summations are performed over the total number of measurements N.This choice of weighting factors minimizes the variance of the estimateof x.

[0142] The third aspect of the invention has numerous advantages. Forone, as with the first and second aspects of the invention, this aspectefficiently utilizes the cellular network, and positioning determinationmethods available therewith, to compute the position of a mobileterminal when the GPS receiver does not have the required number of GPSsatellites in view. Furthermore, the system is efficient because itutilizes the more accurate means for determining location—GPS—wheneverthe requisite number of GPS satellites are available.

[0143] In addition, there is another advantage to switching back to GPSafter the mobile terminal has been using the cellular networkinfrastructure for some period of time to determine location. Byswitching back to GPS, and recalibrating the system, the negative effectof multi-path problems associated with using the TOA and TDOA methodsfor determining location is reduced. That is, after some period of timeof using the TOA or TODA methods to determine location, the mobile mayhave moved out of the region for which the last calibration isappropriate. Recalibrating with the GPS enables any errors due to theuse of the cellular infrastructure signals to be again determined andcorrected.

[0144] By switching back to using GPS as soon as it becomes feasible,the TOA or TDOA methods will be used for the absolute minimum amount oftime, which limits the effect of errors due to multi-path problems.Using both the GPS and cellular signals, an estimate may be made of themulti-path propagation errors in the cellular signals.

[0145] Furthermore, another problem associated with calculating positionbased on cellular network infrastructure is clock drift of the clock inthe mobile terminal. That clock drift can result in erroneous time—andtherefore location—measurements. By switching back to GPS as soon as itbecomes available, the time in which the TOA or TDOA methods are used isrelatively short. Thus, the amount of error due to clock drift, whichincreases the longer the cellular network infrastructure methods areused, can be minimized. In addition, by measuring the rate of clockdrift in the mobile terminal using the GPS time information while theGPS signals are available, a software routine may be implemented, forexample, in the central processor, to compensate for this clock driftwhen GPS is not available at a subsequent point in time.

[0146] Fourth Aspect

[0147] A fourth aspect of the invention may be described in connectionwith the structure shown in FIG. 10, and FIGS. 14-15. In a standardIS-95 CDMA system, the pilot signal component of the CDMA cellular basestation signal can be used to augment and improve the accuracy andavailability of position location using GPS. The CDMA cellular signalcan provide the functional equivalent of the GPS pseudorange. Theadvantage is that this approach requires very little adaptation of thebase station, for example, compared to the second aspect describedabove. The CDMA pilot signal component, such as used in the IS-95standard, has the advantage of being transmitted continuously from eachbase station at a constant power level that enables it to typically bereceived by a mobile terminal from more than one base station. The pilotsignal includes periodic signals that are transmitted at specified timeswith specified offsets. Thus, the time difference of arrival of twopilot signals from two base stations may be readily measured by themobile terminal. The IS-95 standard signalling and control process alsoincludes methods whereby the terminal may be instructed to measure theTDOA of the pilot signals it can receive and to report thesemeasurements to the CDMA control process. Other facets of the IS-95pilot signals and the standard transmissions enable a round trip delay(“RTD”) measurement to be made which may be used to determine themobile's range, or distance, from the serving base station. Either theRTD measurement or a plurality of TDOA measurements may be used toobtain a cellular distance measurement, which represents the distancebetween the mobile and the serving base station.

[0148] In this approach, a base station, such as base station 1035,transmits timing signals to the terminal. In fact, this is already beingdone as part of the regular cellular system transmissions such asaccording to IS-95 or GSM standards. The terminal will receive thesignals at some later time due to the delay in transmission. Theterminal will extract the timing signals from the base station, thoughthose signals will be offset in time due to the propagation delay.Further, the terminal must transmit back to the base station at definedtimes with respect to its receipt of the timing signals from the basestation. For example, the terminal may be required to respond withinplus or minus one microsecond (±1 μs) of its specified transmissiontiming. The base station then receives the return signals from theterminal at some later time due to delay in return transmission. Thetotal delay measured at the base station is the RTD. The radius of thesphere—representing the distance between the base station and theterminal—is one half the RTD times the speed of radio signals in air.This measurement may be generally referred to as a cellular distancemeasurement. The cellular distance measurement, like the cellularposition signal of the third aspect, does not contain GPS informationand rather is independent of GPS.

[0149] Therefore, this distance measurement can be used as a substitutefor a GPS signal when the requisite number of GPS satellite signals arenot in view, or to supplement the information available from GPS so thatboth GPS and cellular signals are used together in order that a moreaccurate position determination may be made than by either systemoperating alone. The advantage of this approach is that the only changeto the base station is the addition of the RTD determination, which canbe done by a processor. FIG. 14 shows a typical base station 1400 with acellular transceiver 1410 for performing conventional functions and aRTD processor 1420 that determines RTD.

[0150] However, errors are present in the above position estimate,largely due to the terminal's internal delays and multi-pathpropagation. While the uncertainty is generally less than 1 microsecond,this translates into an uncertainty in distance of about 150 meters.

[0151] To compensate for this error, GPS can be used. This approach isdepicted in FIG. 15. When a requisite number of reference GPS satellitesare in view, the position of the terminal can be determined. Thereference GPS satellites need not be the same GPS satellites that themobile makes use of at some later point in time when the requisitenumber of GPS satellites are not in view. This determination is shown inblock 1500. The position may be sent to a position processor which maybe located either in the mobile terminal (such as 1025 in FIG. 10) ormay be a server in the network which operates to calculate positions forterminals using the cellular and communications networks. This is shownat block 1510. In addition, it is noted that the locations of the basestations are known. At block 1520 the position processor determines theexpected signal timing using information on the position of the basestation and the position of the mobile as determined using GPS. Forexample, in the case of a system using the RTD measurement processdescribed earlier, the expected RTD would be calculated. Alternatively,for a system using the TDOA measurement process described earlier, theexpected TDOA would be calculated. Then at block 1530 a measurement ismade of the actual signal timing, for example the RTD or the TDOA. Atblock 1540, the difference between the expected and the measured valuesis determined. The difference is stored, at block 1550, as a cellularcorrection term (i.e. a RTD or TDOA correction term) for later use bythe position processor. If the position processor is located in themobile terminal, then the RTD measurements, which are made in the basestation, may be sent to the mobile terminal using the standard messagesignalling facilities of the cellular system. Similarly, if the positionprocessor is connected to the communications network, the TDOAmeasurements made in the terminal may be sent to the position processorusing the standard message signalling facilities of the cellular system.Of course, to determine the position to the best accuracy, the positionprocessor may make use of both TDOA and RTD measurements in itscalculations and need not be restricted to a single measurement type.

[0152] Thereafter, when the requisite number of GPS satellite signalsare not in view at some later point in time, and position is calculatedbased in part using the RTD measured at the base station, or the TDOAmeasured at the terminal, the correction term can be used to reduce theerror due to the unknown delay in the terminal as seen at the servingbase station. It is contemplated that the position location calculationbe done by the network server, or in a suitable processor in theterminal (such as central processor 1015 or position processor 1025 inFIG. 10). In the alternative, the cellular position processor 1025 inthe mobile terminal could measure TDOA of the cellular pilot signals anduse these to supplement the GPS position information.

[0153] It should be noted that other systems (in addition to the IS-95CDMA technique discussed) exist that are capable of measuring the RTD,such as the GSM (European standard) that utilizes TDMA (Time DivisionMultiple Access) cellular techniques. The same concept of utilizingcellular distance measurements as substitutes for missing GPS satellitesignals may be readily applied in such systems. Typically GSM systemsare operated with unknown time offsets in the transmissions from eachbase station. In this case the correction terms, determined through theuse of the GPS calibrations, also compensate for the unknown timeoffsets of the GSM base station transmissions.

[0154] It should be understood that while the four aspects of theposition location system of the present invention have each beendescribed, and may be used, separately, two or more aspects may becombined in a single position location system. For example, the DGPSerror correction information may be provided to the mobile terminalthrough the cellular network according to the first aspect of theinvention, and in that same system, a base station of the cellularnetwork may also provide a pseudosatellite signal. In such a system, thecontrol processing unit 940 would utilize the error correction data andcorrect the GPS pseudoranges to obtain corrected GPS ranges. Thus, theDGPS processor would essentially be part of the control processing unit.Of course, a separate DGPS processor could be used instead.

[0155] Alternatively, the first and fourth aspects could be combined.DGPS error correction data would be provided to the mobile terminal andthe base station could provide the CDMA cellular signals that providethe functional equivalent of the GPS satellite signal. Anotheralternative is that the DGPS error correction information be providedthrough the cellular network infrastructure in a system that alsoswitches between using GPS and the cellular network infrastructure todetermine location.

[0156] Furthermore, the position location system may have the second,third, and fourth aspects of the invention available, and the systemmerely determines which aspect to utilize when the requisite number ofGPS satellites are not in view of the GPS receiver. The selection of thedesired approach could be carried out, for example, by a processor suchas the central processor 1015 of the mobile terminal, shown in FIG. 10.Finally, all four aspects may be combined in a single position system aswell. Therefore, the present invention is not limited to using only oneaspect of the invention in a single position location system. Rather,two or more aspects of the invention can be used in the same positionlocation system.

[0157] In addition, changes to the structures as presented above do notdepart from the scope of the present invention. For example, a mobileterminal may contain both the GPS and cellular portions in the samemobile unit, or each component can be housed separately with a relevantmeans for communication between the two. In that regard, the term“mobile terminal” may refer to a terminal containing either a GPSreceiver or a cellular mobile terminal, or both. Furthermore, many ofthe individual components may be combined into a single unit with othercomponents. For example, the central processor in the third aspect ofthe invention may be contained in either the GPS receiver portion or themobile cellular portion.

[0158] Furthermore, substitute components that provide substantially thesame functions as those disclosed also do not depart from the scope ofthe invention. For example, alternate structures known in the art forconverting a received signal into an intermediate frequency (IF) may beused rather than the structure disclosed herein. Finally, the generalcomponents shown for the mobile terminals, base stations, and MTSO donot necessarily indicate that this is the only structure present inthese items. Rather, the illustration of certain components is intendedfor an easier understanding of the present invention. Finally, while theconnections in the Figures, for example between the DGPS processor andthe GPS receiver in FIG. 5, are shown as electrical connections, itshould be understood that other connections are possible, such asoptical connections.

We claim:
 1. A position location system for determining a geographicposition comprising: a plurality of cellular network base stationspositioned at predetermined locations; and a communications network thatcommunicates with said plurality of cellular network base stations,wherein at least one of said plurality of cellular network base stationscomprises a generator that generates a global positioning systempseudosatellite signal independent of receiving global positioningsystem satellite signals.
 2. The position location system of claim 1 ,wherein said generator comprises: a global positioning system processorthat generates a pseudo global positioning system data signal containingdata representing a location of said one of said plurality of cellularnetwork base stations, a coarse acquisition code portion that generatesa coarse acquisition encoding signal, and a modulator connected to saidglobal positioning system processor and to said coarse acquisition codeportion, wherein said modulator combines said pseudo global positioningsystem data signal and said coarse acquisition encoding signal toproduce said global positioning system pseudosatellite signal.
 3. Theposition location system of claim 2 , wherein said one of said pluralityof cellular network base stations further comprises a converterconnected to said modulator, wherein said converter converts said globalpositioning system pseudosatellite signal into a radio frequency signalhaving a frequency in a cellular frequency band.
 4. The positionlocation system of claim 3 , wherein said one of said plurality ofcellular network base stations further comprises a level adjusterconnected to said converter, wherein said level adjuster dynamicallyadjusts an amplitude level of said radio frequency signal to apredetermined level relative to transmitted cellular signals.
 5. Theposition location system of claim 4 , wherein said one of said pluralityof cellular network base stations further comprises: a cellular radiothat transmits said transmitted cellular signals, and a combinerconnected to said cellular radio and to said level adjuster, whereinsaid combiner combines said cellular signals and said radio frequencysignal into a combined cellular signal.
 6. The position location systemof claim 5 , wherein said one of said plurality of cellular network basestations transmits said combined cellular signal to a mobile terminal.7. The position location system of claim 1 , further comprising a mobileterminal positioned in a transmitting vicinity of said one of saidplurality of cellular network base stations, wherein said one of saidplurality of cellular network base stations transmits said globalpositioning system pseudosatellite signal to said mobile terminal. 8.The position location system of claim 7 , wherein said mobile terminalcomprises: a cellular antenna that receives a radio frequency signalhaving a frequency in a cellular frequency band, said radio frequencysignal containing differential error correction data; a radio interfaceunit connected to said cellular antenna, wherein said radio interfaceunit converts said received radio frequency signal into an intermediatefrequency signal; a digital signal processor connected to said radiointerface unit, wherein said digital signal processor converts saidintermediate frequency signal into a data stream; a central processorconnected to said digital signal processor, wherein said centralprocessor extracts said differential error correction data from saiddata stream; and a differential global positioning system processorconnected to said central processor, wherein said central processorforwards said differential error correction data to said differentialglobal positioning system processor.
 9. A position location system fordetermining a geographic position comprising: a cellular antenna thatreceives a cellular signal having a frequency in a cellular frequencyband, wherein said cellular signal contains global positioning systempseudosatellite data; a first converter connected to said cellularantenna, wherein said first converter converts said received cellularsignal into a first intermediate frequency signal having a firstfrequency; a global positioning system receiver antenna that receivesglobal positioning system satellite signals having frequencies in asatellite frequency band from a plurality of global positioning systemsatellites; a second converter connected to said global positioningsystem receiver antenna, wherein said second converter converts saidreceived global positioning system satellite signals into a secondintermediate frequency signal having a second frequency, wherein saidfirst frequency of said first intermediate frequency signal issubstantially equal to said second frequency of said second intermediatefrequency signal; a selector connected to said first converter and tosaid second converter, wherein said selector selects one of said firstintermediate frequency signal and said second intermediate frequencysignal; and a control processing unit connected to said selector, saidcontrol processing unit configured to utilize said first intermediatesignal and said second intermediate signal when a requisite number ofglobal positioning system satellites are not in view of the globalpositioning system receiver antenna, to calculate global positioningsystem pseudoranges.
 10. The position location system of claim 9 ,further comprising a mobile terminal that houses said cellular antenna,said first converter, said global positioning system receiver antenna,said second converter, said selector, and said control processing unit.11. The position location system of claim 10 , further comprising: aplurality of cellular network base stations positioned at predeterminedlocations; and a communications network that communicates with saidplurality of cellular network base stations, wherein at least one ofsaid plurality of cellular network base stations comprises a generatorthat generates said cellular signal containing said global positioningsystem pseudosatellite data, and wherein said one of said plurality ofcellular network base stations transmits said cellular signal to saidmobile terminal.
 12. The position location system of claim 9 , whereinsaid cellular signal further contains differential error correctiondata, and wherein said control processing unit is further configured tocorrect said global positioning system pseudoranges using saiddifferential error correction data, to obtain corrected globalpositioning system ranges.
 13. The position location system of claim 9 ,further comprising an amplitude controller connected to said firstconverter, wherein said amplitude controller adjusts an amplitude ofsaid first intermediate frequency signal before said first intermediatefrequency signal is forwarded to said selector.
 14. A method fordetermining a geographic position of a mobile terminal comprising thesteps of: receiving global positioning system satellite signals havingfrequencies in a satellite frequency band at the mobile terminal from aplurality of global positioning system satellites that are in view ofthe mobile terminal; converting said global positioning system satellitesignals into a first intermediate frequency signal; receiving a cellularsignal having a frequency in a cellular frequency band at the mobileterminal, wherein said cellular signal contains global positioningsystem pseudosatellite data; converting said cellular signal into asecond intermediate frequency signal; and calculating the geographicposition using both said first intermediate frequency signal and saidsecond intermediate frequency signal when a requisite number of globalpositioning system satellites are not in view of the mobile terminal.15. The method of claim 14 , further comprising the step of transmittingsaid cellular signal from a cellular network base station to the mobileterminal.
 16. The method of claim 15 , further comprising the step ofgenerating said cellular signal in the cellular network base station,wherein said generating step comprising the steps of generating a pseudoglobal positioning system data signal containing data representing alocation of the cellular network base station, generating a coarseacquisition encoding signal, combining said pseudo global positioningsystem data signal and said coarse acquisition encoding signal into apseudosatellite signal, and converting said pseudosatellite signal toproduce at least a part of said cellular signal.
 17. A method fordetermining a geographic position of a mobile terminal comprising thesteps of: receiving global positioning system satellite signals havingfrequencies in a satellite frequency band at the mobile terminal from aplurality of global positioning system satellites that are in view ofthe mobile terminal; receiving a cellular signal having a frequency in acellular frequency band at the mobile terminal, calculating a cellulardistance measurement representing a distance between the mobile terminaland a cellular network base station using said cellular signal;calculating the position of the mobile terminal using said receivedglobal positioning system signals when a requisite number of globalpositioning system satellites are in view of the mobile terminal; andcalculating the position of the mobile terminal using said receivedglobal positioning system signals and the cellular distance measurementwhen the requisite number of global positioning system satellites arenot in view of the mobile terminal.
 18. The method of claim 17 , whereinsaid received cellular signal is a code division multiple accesscellular signal.
 19. The method of claim 17 , wherein said receivedcellular signal is a time division multiple access cellular signal. 20.The method of claim 17 , wherein said step of calculating the cellulardistance measurement utilizes a round trip delay measurement.
 21. Themethod of claim 17 , wherein said step of calculating the cellulardistance measurement utilizes a plurality of time difference of arrivalmeasurements.
 22. A method for determining a geographic position of amobile terminal comprising the steps of: receiving global positioningsystem satellite signals having frequencies in a satellite frequencyband at the mobile terminal from a plurality of global positioningsystem satellites that are in view of the mobile terminal; receiving acellular signal having a frequency in a cellular frequency band at themobile terminal, calculating a cellular distance measurementrepresenting a distance between the mobile terminal and a cellularnetwork base station using said cellular signal; correcting saidcellular distance measurement using a cellular correction term, toobtain a corrected cellular distance measurement; calculating theposition of the mobile terminal using said received global positioningsystem signals when a requisite number of global positioning systemsatellites are in view of the mobile terminal; and calculating theposition of the mobile terminal using said received global positioningsystem signals and said corrected cellular distance measurement when therequisite number of global positioning system satellites are not in viewof the mobile terminal.
 23. The method of claim 22 , wherein saidcellular correction term is a round trip delay correction term that iscalculated by the steps comprising: determining an expected round tripdelay between the mobile terminal, whose position is determined usingreference global positioning satellites, and said cellular network basestation, whose position is known; determining an actual round trip delaybetween the mobile terminal and said cellular network base station;calculating a difference between said expected round trip delay and saidactual round trip delay; and storing the calculated difference as saidround trip delay correction term.
 24. The method of claim 22 , whereinsaid received cellular signal is a code division multiple accesscellular signal.
 25. The method of claim 22 , wherein said receivedcellular signal is a time division multiple access cellular signal. 26.The method of claim 22 , wherein said cellular correction term is a timedifference of arrival correction term that is calculated by the stepscomprising: determining an expected time difference of arrival betweenthe mobile terminal, whose position is determined using reference globalpositioning satellites, and said cellular network base station, whoseposition is known; determining an actual time difference of arrivalbetween the mobile terminal and said cellular network base station;calculating a difference between said expected time difference ofarrival and said actual time difference of arrival; and storing thecalculated difference as said time difference of arrival correctionterm.